The invention disclosed herein relates to a system for controlling a plurality of positioning devices such as d.c. motors in apparatus such as an electronic postage meter, and to controlling other functions of the apparatus. More particularly, the invention relates to an electronic postage meter including a plurality of value settable print wheels, and a microcomputer for controlling multiple d.c. motors to set the print wheels and for controlling other postage meter functions. The invention also relates to an incremental position encoder for supplying information related to the actual positions of devices such as print wheels which are to be positioned. The invention has particular application to devices such as electronic postage meters requiring low cost, compact apparatus for controlling positioning of a plurality of devices such as postage value digit devices, e.g., print wheels.
Relatively accurate positioning of devices such as a print wheel in an electronic postage meter may be obtained using a stepper motor or a d.c. motor. An advantage of a stepper motor over a d.c. motor is that the stepper motor has defined stable equilibrium or detent positions, i.e., discrete positions in a fixed increment or step angle, which allows the stepper motor to be driven in open loop mode. As a result, control of a stepper motor as compared to a d.c. motor is relatively simple and does not require a motion transducer, such as an encoder, and a control loop, although it may be desirable to verify the detent positions of the motor. However, under high inertial loads and opposing load torques, for example, friction, a stepper motor may be unstable and lose step synchronism. To avoid such instability, the size (torque) of the stepper motor is increased in comparison to load requirements to provide sufficient margins at the desired speeds. Thus, a disadvantage of a stepper motor in positioning apparatus in comparison to a d.c. motor is the relatively large size-to-torque ratio needed to ensure stable operation.
An advantage of a d.c. motor in positioning apparatus in comparison to a stepper motor is that the d.c. motor may be smaller for the same load requirements and yet operate at higher speed than a larger size stepper motor. Additionally, the smaller d.c. motor allows a higher load-to-motor coupling (gear) ratio than a larger size stepper motor for the same load. Thus, the d.c. motor increases the torque available to the load at the desired speed as compared to a stepper motor which at the same torque and coupling ratio will operate at lower speed. Other advantages of a d.c. motor over a stepper motor are the reduced power dissipation (and corresponding reduced heat generation) and reduced power supply requirement for d.c. and stepper motors of comparable size-to-torque ratio. A d.c. motor has a single phase winding with a high back electromotive force, and requires a single output stage driver for the single phase winding. A comparable stepper motor has a two phase winding with the same winding resistance of the single phase d.c. motor winding. Hence, the stepper motor winding requires twice the power supply requirement as the comparable d.c. motor. Also, the stepper motor requires an output stage driver for each winding. Hence, the stepper motor requires twice the number of output stage drivers as the d.c. motor.
A disadvantage of a d.c. motor over a stepper motor is that the d.c. motor requires a control loop, which adds to the cost and complexity of the positioning apparatus.
If multiple loads are to be controlled by separate motors, the disadvantages of stepper motors and d.c. motors for position control implemented as described above are multiplied.
The disclosure of the following U.S. patents are incorporated herein by reference: U.S. Pat. Nos. 4,630,210, 4,631,681, 4,636,959, 4,646,635, 4,665,353, 4,638,732 and 4,774,446 all of Salazar et al., and 4,635,205 of Eckert, et al. All of those patents are assigned to the assignee of this application. Those patents disclose an electronic postage meter including a computer-controlled d.c. motor used to control a plurality of mechanical loads, for example print wheels. A stepper motor and gearing selectively couple the same d.c. motor to a plurality of loads. The time-intensive control function for the d.c. motor is implemented by the computer and software, except for the amplifier driving the d.c. motor and an actual position encoder and counting circuitry which feeds back actual positional information. The computer and the software calculate and apply to the d.c. motor, via the amplifier, pulse width modulated (PWM) drive signals utilizing a digital compensator derived from an analysis of the d.c. motor, the motor load, and other control loop components. The d.c. motor is driven according to a predetermined velocity versus time profile stored in the computer. The encoder and counting circuitry provide digital signals to the computer and the computer provides digital PWM drive signals so that analog-to-digital and digital-to-analog converter devices are not required in the controller electronics.
In the above patents, the computer is dedicated to motor control and separate microprocessors are provided to carry out other postage meter system functions such as keyboard and display control, accounting and printing.
In the patents referenced above, the actual position encoder is an incremental encoder which provides in response to motor shaft rotation two electrical signals 90 electrical degrees out of phase, i.e., in quadrature. The two signals together provide N quadrature states per motor shaft rotation. Shaft position is determined by counting the quadrature states and rotation direction is determined from the phase of the two signals, i.e., which signal leads (or lags) the other. In the patents referenced above, the incremental encoder is implemented by a transparent disk having a plurality of opaque lines formed at equidistantly angularly-spaced intervals along one of the disc's opposed major surfaces, and an optical sensing device for serially detecting the presence of the respective opaque lines as they successively pass reference positions. In response to detecting the presence of the opaque lines, the encoder provides two output signals on two separate lines or channels in quadrature.
The two channel, quadrature output of the incremental encoder is decoded by counting circuitry. If desired, the quadrature signals may be decoded by the computer. Whether an external circuit or the computer decodes the quadrature output signals of the encoder depends upon whether the computer's internal counting circuit is available or is being used for other purposes. External decoding circuits are currently available as monolithic integrated circuits, for example, DHC 2000 from Texas instruments Incorporated.
To achieve high positional accuracy, position controllers of the type described above utilize a high resolution encoder and sample at 1 ms or less intervals, which means that the controllers have a relatively high motor control bandwidth requirement of 1 KHz or greater. High bandwidth controllers utilizing high resolution encoders are relatively expensive.
Because of the high bandwidth requirement, a processor in such controllers is typically dedicated to motor control. Therefore, larger systems which include a d.c. motor and loop control therefor require other processors for control of other system functions, which increases the size and cost of the larger system.
U.S. Pat. Nos. 4,710,883 of Wilson et al., 4,710,882 of DiGiulio et al., and 4,701,856 of DiGiulio et al., all assigned to the assignee of this application, disclose an electronic postage meter comprising a microcomputer for controlling postage meter functions including postage value setting, postage printing and postage accounting. The disclosures of the '883, '882 and '856 Patents are incorporated herein by reference. As disclosed in these patents, a single central processing unit (CPU) performs calculations and data flow within the postage meter and controls postage setting and printing. The '883 Patent discloses that the electronic postage meter therein includes a timer, preferably external to the microcomputer, for providing a periodic interrupt in response to which the microcomputer interrupts a routine being executed and proceeds to examine sensors, inputs and outputs for machine status changes. A queue of individual tasks which comprise the operation of the postage meter are scheduled as required in accordance with machine status. A task switcher proceeds to service tasks in a "round-robin" fashion. Tasks that are inactive or waiting for an event are ignored. The first task encountered that needs to be serviced is granted control of the CPU. When all tasks have been polled, the polling cycle begins again. The tasks quickly perform a function and then relinquish the CPU to wait for an event to occur. This permits the next task in the round-robin to gain the services of the CPU. A task may run to completion or suspend itself and permit task switching. A timer is set to zero each time that a task is activated, and then decremented in 2.5 ms intervals during CPU servicing of that task. Task priorities are assigned.
The '883, '882 and '856 Patents do not provide specific details of the postage setting and printing apparatus.
U.S. Pat. No. 4,731,728 of Muller, also assigned to the assignee of this application, discloses a postage meter having separate microprocessors for controlling postage value selection, accounting and keyboard and display functions. This patent also discloses shutter and interposer apparatus, and an accounting algorithm for controlling printing and postage accounting. The disclosure of the '728 Patent is incorporated herein by reference.
There is thus a need for a low cost, compact system, both mechanically and electrically, for accurately positioning a plurality of devices such as print wheels in an electronic postage meter, and which, when forming part of a larger system such as a postage meter, carries out other system functions, for example, postage meter keyboard and/or display control, and/or accounting and/or printing functions. There is also a need for a low cost, low bandwidth position encoder for use in a close loop position control system, particularly a low bandwidth system.